My 2¢ but you do want pads on both sides.... I never expect to fall on my left since I land with my right leg, but it has happened enough that I usually wear both sides when I get padded up - without fail I will catch an edge when I am not expecting it and go splat on the unpadded side - probably because I am so focused on what I am doing I am not watching the ice as closely for ruts and holes and stuff
Anyways, my personal preference is homemade pads made from the "blue foam" camping pads.. it's closed cell foam, 1/2" thick, and it's really lightweight, and you can cut it with any normal scissors. REI (if you have one nearby?) sells a chair sized piece of the foam for $8 and it's more than enough to cut a set of hip pads from (as well as tailbone, palm, whatever you want). I think you can get a regular length sleeping pad from somewhere like Walmart or Academy for about $15, but I think the quality of the REI foam is a little better. Like waxel pads they can just be slipped into a fitted pair of pants or under tights and they will stay in place just fine.
I'm not sure about the size/usefulness of the MSR pads. If you could find out the exact dimensions that would be helpful, but I would imagine they are not as large as you'd need. Motocross pads are meant to protect the front and upper part of the iliac crest... they aren't quite used the same way a skater would use them (they go in the motocross pants in about the same orientation as front pockets would). The pads in my 10yo's pants are barely bigger than my hands, and I don't have big hands.